An on-line monitoring technology of metal oxide arrester（MOA）degradation based on the particle swarm optimization algorithm is proposed for on-line aging monitoring of MOA. PSO algorithm is employed to compute the aging parameters

α、k and c of the MOA equivalent model for monitoring the degradation of MOA. In addition

the simulation by the MATLAB to analysis the impact of the voltage harmonic in the power grid to the algorithm in this paper. Simulation shows that PSO algorithm can fit the calculated leakage current of the equivalent model to the measured one

and calculate the parameters α

k and c

which change during the aging period of MOA

for monitoring. It is concluded that the proposed on-line aging monitoring technology can suppress the influences of the content and initial phase of working voltage harmonics to raise its accuracy

and is effective and reliable.